Every time I launch NS2, I get the message from DirectX, "Performing first time setup". This is annoying because (among other things) I have to enter the Admin password every time. NS2 is the only game that does this.

This started happening after an NS2 update some years ago. After having this problem despite different computers, different OS's, and different installs, I seem to have finally found a fix.

Delete: "InstallScript.vdf"

I found it in the directory: ...\Steam\steamapps\common\Natural Selection 2

Apparently, this triggers Steam to run "%INSTALLDIR%\\redist\\DirectX\\DXSETUP.exe" every time the game is launched. Afterwards I revalidated local game files, but it did not trigger the file to be reinstalled.

This is reportedly an old Steam issue, and may apply to other games as well.

I learned about this from a stale 2013 thread:
performing the first time setup? every time?